This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-055041, filed Feb. 28, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of zero adjustment of a load cell for detecting back pressure of a screw in an injection-unit of an electric injection molding machine.
2. Description of the Related Art
In an electric injection molding machine, a given amount of molten resin is stored in a heating barrel of an injection unit and then a screw is moved forward in the heating barrel to inject the molten resin into a mold. When the molten resin is stored in the heating barrel and when it is injected into the mold, the pressure of the molten resin in the heating barrel is detected using a load cell mounted on a rear end portion of the screw. This pressure is called back pressure of the screw.
Since zero point of a load cell is easy to vary with time, it needs to be adjusted appropriately in order to exactly control the back pressure of the screw. Usually, the zero adjustment of the load cell is carried out periodically. Beside this, if an output of the load cell exceeds a given allowance when the heating barrel is empty and evidently the screw is under no-load conditions, e.g., when the injection molding machine starts to operate, it is determined that the zero point of the load cell is shifted and its adjustment is carried out at any time.
Jpn. Pat. Appln. KOKAI Publication No. 2000-6217 discloses the method of zero adjustment of a load cell as follows. A plurality of advance and retreat operations of a screw are carried out. An average value of outputs of the load cell is calculated for each of the advance and retreat operations and an average of the average values of outputs of the load cell for the plurality of advance and retreat operations is taken, thereby performing zero adjustment of the load cell.
Jpn. Pat. Appln. KOKAI Publication No. 9-117946 discloses the method of zero adjustment of a load cell as follows. After a screw is moved to the half stroke position between the limit of advance and that of retreat, a servomotor is damped and oscillated to stop the screw in the half stroke position. Thus, a ball screw that drives the screw causes a backlash having equal play in both advance and retreat directions and a load acting on the load cell is removed, thereby performing zero adjustment of the load cell.
The foregoing prior art methods of zero adjustment of a load cell had problems of requiring a complicated procedure and a lot of time to perform an adjustment operation.
The present invention has been developed in consideration of the above problems of a prior art method of zero adjustment of a load cell for detecting back pressure of a screw in an electric injection molding machine. An object of the present invention is to provide a method capable of performing zero adjustment of a load cell in a short time by a simple procedure.
According to the present invention, there is provided a method of zero adjustment of a load cell for detecting back pressure of a screw in an electric injection molding machine, the electric injection molding machine comprising:
a heating barrel;
a screw incorporated in the heating barrel and driven by a servomotor in an axial direction, which injects molten resin into a mold; and
a load cell connected to a rear end portion of the screw, which detects back pressure of the screw,
wherein the method comprises the steps of:
driving the servomotor for a preset period of time in a first direction in a half stroke position of the screw, then stopping the servomotor by automatic decompression, and detecting and recording an output of the load cell as a first output
driving the servomotor for a preset period of time in a second direction opposite to the first direction, then stopping the servomotor by automatic decompression, and detecting and recording an output of the load cell as a second output; and
calculating an average of the first output and the second output and performing zero adjustment of the load cell using the average while considering the average to be an output of the load cell under no-load conditions.
In the above-described zero adjustment method, the xe2x80x9cautomatic decompressionxe2x80x9d means that the servomotor is stopped by gradually reducing a current supplied to the servomotor. The automatic decompression is performed, for example, by reducing a current linearly or exponentially.
Since the servomotor is stopped by automatic decompression as described above, the influence of the friction in the ball screw for driving the screw and the resin resistance in the heating barrel can be reduced as much as possible. If, therefore, the outputs of the load cell are detected after the servomotor advances and retreats under the above conditions and then an average of the outputs is calculated, an output of the load cell under no-load conditions is exactly obtained, and zero of the load cell can be adjusted using the average.
Preferably, the heating barrel is retreated and separated from the mold prior to the step of driving the servomotor for a preset period of time in the first direction.
Preferably, when the average of the first output and the second output exceeds a given tolerance, it is determined that the load cell malfunctions, and a control system issues an alarm.
Preferably, date and time when the zero adjustment of the load cell is performed and the average of the first output and the second output are stored in a storage unit as actual-result data, and a given number of past actual-result data is displayed on a display panel of a control system.